Relex 7
Visual Reliability Software
Reference Manual
Relex Software Corporation 540 Pellis Road Greensburg, PA 15601 USA
724-836-8800 Fax 724-836-8844
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Table of Contents
Chapter 1 Getting Started
1-1
Read Me First!...1-1 Welcome...1-1 Installation...1-1 Licensing Relex...1-4 Starting Relex...1-5 Converting from Previous Versions of Relex...1-5 Basic User Interface...1-8 Relex Documentation...1-9 Documentation Conventions...1-10 Where to Find Help...1-11 Introduction to Relex and Reliability...1-12 Who, What, and Why?...1-13 Reliability Techniques...1-15 Introduction to Reliability Prediction...1-17 Introduction to Reliability Block Diagram (RBD)...1-21 Introduction to FMEA...1-22 Introduction to Fault Tree...1-24 Introduction to Maintainability...1-25 Introduction to Life Cycle Cost (LCC)...1-26
Chapter 2 General
2-1
Relex Basics...2-1 Starting Relex...2-1 Tip of the Day...2-1 Relex Desktop...2-2 Right Mouse Support...2-3 Guide Conventions...2-3 Mouse Conventions...2-3 Menu Conventions...2-4 Keyboard Options...2-4 File Management...2-6 Creating Files...2-6 Opening Files...2-7 Saving Files...2-8 Closing Files...2-8 Copying Files...2-9 Renaming Files...2-9 Moving Files...2-9 Options...2-9 Library Searches Tab...2-10 Settings Tab...2-11 Display Tab...2-13 Library Updates Tab...2-14
Behavior Tab...2-18 Fault Tree Tab...2-20
Chapter 3 File Types
3-1
Introduction...3-1 Project Files...3-1 Sample Project File...3-2 Creating/Opening a Project File...3-2 Project File Window...3-13 System Tree...3-14 Parts Table...3-16 Life Cycle Cost Files...3-32 Input Format Files...3-32 Input Format Files...3-32 Creating/Opening an Input Format File...3-33 Report Design Files...3-35 Reliability Prediction Value-Added Files...3-35 FMEA Value-Added Files...3-35 Fault Tree Value-Added Files...3-36 Maintainability Value-Added Files...3-36 Life Cycle Cost Value-Added Files...3-37 Miscellaneous Files...3-37
Chapter 4 Reporting and Graphing
4-1
Reporting...4-1 Generating a Report in Relex...4-1 Printing a Report to a Printer...4-3 Generating a Report to a File...4-4 Relex Report Designer...4-6 Relex RTF Editor...4-22 Relex Text Editor...4-24 Graphing...4-25 Graph Wizard...4-25 Editing a Graph...4-31 Printing a Graph...4-33 Creating a Custom Graph...4-33 Saving a Graph File...4-34
Chapter 5 Importing and Exporting
5-1
Introduction to the CAD Interface...5-1 Supported File Types...5-1 Relex Files that Support Importing or Exporting...5-1 Types of Files that Can Be Imported or Exported...5-2 Importing and Exporting Procedure...5-4 Importing with the Relex CAD Interface...5-4 Exporting with the Relex CAD Interface...5-6 Relex Import/Export Log...5-8 Importing and Exporting Relex Files...5-8 Importing Data to a Project File...5-8
Importing and Exporting FMEA Data...5-13 Importing and Exporting Maintainability Data...5-13 Importing and Exporting a Parts Library File...5-14 Importing and Exporting a Correlation File...5-14 Importing and Exporting an LCC File...5-15 Importing and Exporting Fault Tree Data...5-15 Importing and Exporting Other File Types...5-16 Importing Thermal Data from a Thermal Analysis Program...5-16 Importing Stress Data from a Stress Analysis Program...5-17 Exporting LSAR Data to a Logistics Program...5-18 Importing Relex Files from an Older Version of Relex...5-18 Importing RPP Files...5-18 Importing and Exporting Mentor Graphics Files...5-18 Importing and Exporting Details...5-18 Importing and Exporting Delimited Text Files...5-19 Importing and Exporting BOM Text Files...5-19 Notes Regarding Importing and Exporting Keyword Files...5-20 Importing and Exporting Spreadsheet Files...5-21 Importing and Exporting Database Files...5-22 Importing an Entire Project File...5-22 Special Notes...5-24 Advanced Importing and Exporting...5-25 Settings File...5-25 Format File...5-25 Advanced Importing...5-25 Advanced Exporting...5-27
Chapter 6 Common Features
6-1
Macro Function...6-1 Document Interface Macro...6-1 Command Macro...6-1 Macro Editor...6-1 Supplied Example Macros...6-2 Macro Browse Feature...6-2 Running a Macro...6-3 Debugging...6-3 Checking Syntax...6-4 Stopping a Macro...6-4 Important Notes Regarding Macros...6-4 Find, Replace, & Spell Check...6-5 Find...6-5 Search and Replace...6-5 Spell Check...6-6 Find/Replace/Check Spelling Windows...6-8 Revision Control...6-10 Notes Regarding Revision Control...6-11 Opening a File as Exclusive...6-11 Password Protection...6-11 Assigning a Password...6-12
Chapter 7 Reliability Prediction
7-1
Introduction...7-1 Getting Started with Reliability Prediction...7-2 How Do You Begin a Reliability Prediction Analysis?...7-2 What Are the Main Steps in Performing a Reliability Prediction in Relex?...7-3 Is the Reliability Prediction Component Oriented or Will COTS Assemblies be Used?...7-5 Which Parts or Assemblies Will Have Calculated Failure Rates, and Which Will Have Specified Failure Rates?...7-5 Reliability Prediction Reference...7-6 Relex Sample Reliability Prediction File...7-6 Creating/Opening a Reliability Prediction File...7-6 Entry of Part Lists...7-6 Entry of Part Data...7-9 NPRD95 Parts...7-12 Hybrids...7-14 Reliability Prediction Value-Added Files...7-15 Global Changes...7-34 Building and Updating...7-37 Reliability Prediction Calculations...7-40 The Equations...7-41 Entering 217 General Integrated Circuits...7-41 Entering 217 VHSIC/VLSI Integrated Circuits...7-43 Entering 217 GaAs Integrated Circuits...7-44 Entering 217 Bubble Memories...7-45 Entering 217 SAWs...7-46 217 Quality Levels...7-46 217 Temperature Fields...7-48 217 Semiconductors...7-52 Entering 217 Diodes...7-52 Entering 217 General Transistors...7-53 Entering 217 Microwave Transistors...7-55 Entering 217 High Frequency Transistors...7-56 Entering 217 General Optoelectronic Devices...7-57 Entering 217 Laser Diodes...7-57 Entering 217 Fixed Resistors...7-58 Entering 217 Variable Resistors...7-60 Entering 217 Capacitors...7-62 Entering 217 Inductive Devices...7-63 Entering 217 Rotating Devices...7-64 Entering 217 Relays...7-65 Entering 217 Switches...7-67 Entering 217 Standard Connectors...7-68 Entering 217 Boards with Plated through Holes...7-70 Entering Other 217 Connectors...7-70 Entering SMT Interconnect Assemblies...7-71 Entering 217 Miscellaneous Parts...7-73 Entering User-Defined Parts...7-74 Entering Software Reliability...7-76 Entering 217 Tubes...7-77
Entering Telcordia (Bellcore) Integrated Circuits...7-81 Entering Telcordia (Bellcore) Discrete Semiconductors...7-82 Entering Telcordia (Bellcore) Resistors...7-83 Entering Telcordia (Bellcore) Capacitors...7-84 Entering Telcordia (Bellcore) Inductors...7-84 Entering Telcordia (Bellcore) Rotating Devices...7-84 Entering Telcordia (Bellcore) Relays...7-84 Entering Telcordia (Bellcore) Switches...7-84 Entering Telcordia (Bellcore) Connections...7-85 Entering Telcordia (Bellcore) Miscellaneous Parts...7-85 Telcordia (Bellcore) Other Components...7-85 Entering Mechanical Parts...7-85 Entering Seals...7-86 Entering Springs...7-89 Entering Solenoids...7-92 Entering Bearings...7-93 Entering Gears and Splines...7-95 Entering Filters...7-97 Entering Poppets and Spools...7-99 Entering Pistons and Cylinders...7-101 Entering Shafts...7-106 Entering Fluid Drivers...7-108 Entering Brake and Clutch Friction Linings...7-109 Entering Housings and Casings...7-110 Entering Compressor Diaphragms...7-111 Entering Electric Motor Windings...7-111 Entering Motor Brushes...7-112 Entering Threaded Fasteners...7-112 Entering Sensors and Transducers...7-114 Calculation Methods...7-116 Performing a Reliability Prediction Calculation...7-118 Reliability Prediction Calculation Results...7-122 Generating Reports in Relex Reliability Prediction...7-122
Chapter 8 Reliability Block Diagram (RBD)
8-1
Introduction...8-1 Getting Started with RBD...8-2 What is the Goal of an RBD?...8-2 What Are the Main Steps for Creating an RBD in Relex?...8-3 What Decisions Must be Made to Determine How to Structure Your RBD?...8-5 Improving System Reliability and Availability Using the Relex RBD...8-6 RBD Reference...8-8 Sample RBD Files...8-8 Creating/Opening an RBD...8-9 Adding Figures to an RBD...8-10 Editing the RBD Layout...8-12 Assigning Properties to Figures in the RBD...8-16 Connecting Figures...8-27 Adding Labels to an RBD...8-30
Managing Multiple Figures and RBDs...8-33 RBD Calculations...8-35 Performing a Reliability Block Diagram Calculation...8-35 Reviewing RBD Calculation Results...8-38 Monte Carlo Simulation...8-38 RBD Calculation Equations...8-39 Spares Optimization Calculations...8-45 Generating Reports in Relex RBD...8-48
Chapter 9 FMEA
9-1
Introduction...9-1 Getting Started with FMEA...9-1 What is a FMEA?...9-1 What is the Goal of a FMEA?...9-2 How Do You Begin a FMEA?...9-2 What Are the Main Steps in Performing a FMEA?...9-4 What Decisions Must be Made to Determine How to Structure Your FMEA?...9-6 FMEA Reference...9-6 Relex Sample FMEA File...9-6 Creating/Opening a FMEA File...9-7 Entering Data on the FMEA Data Tab...9-7 Levels of Data in Relex FMEA...9-8 Entering FMEA Data...9-8 FMEA and Fault Tree...9-12 FMEA Value-Added Files...9-12 FMEA Calculations...9-20 Performing a FMEA Calculation...9-20 FMEA Calculation Results...9-20 FMEA Calculations and Equations...9-21 Graphing FMEA Calculation Results...9-23 Generating Reports in Relex FMEA...9-26
Chapter 10 Fault Tree
10-1
Introduction...10-1 Getting Started with Fault Tree...10-1 What Is the Goal of a Fault Tree?...10-1 How Do You Begin a Fault Tree Analysis?...10-2 What Are the Main Steps in Creating a Fault Tree?...10-2 Fault Tree Reference...10-3 Sample Fault Tree Files...10-3 Creating/Opening a Fault Tree...10-4 Fault Tree Gates and Events...10-5 Inserting Gates and Events to a Fault Tree...10-9 Gate and Event Types and Styles...10-11 Editing the Fault Tree Layout...10-12 Linking a Fault Tree to an RBD...10-13 Gate and Event Properties...10-14 Connector Properties...10-17 Inserting Labels...10-18
Managing Large Fault Trees...10-20 Relex Fault Tree Value-Added Files...10-23 Fault Tree Calculations...10-29 Performing a Calculation...10-29 Calculation Results...10-33 Fault Tree Calculation Equations and Theory...10-34 Generating Reports in Relex Fault Tree...10-37
Chapter 11 Maintainability Prediction
11-1
Introduction...11-1 Getting Started with Maintainability Prediction...11-1 What is the Goal of a Maintainability Prediction?...11-1 How Do You Begin a Maintainability Prediction?...11-2 What Are the Main Steps for Performing a Maintainability Prediction in Relex?..11-3 What Decisions Must be Made to Determine How to Structure Your Maintainability Prediction?...11-5 Maintainability Prediction Reference...11-6 Relex Sample Maintainability File...11-6 Creating/Opening a Maintainability Prediction File...11-6 Entering Data on the Maintainability Data Tab...11-7 Maintainability Value-Added Files...11-8 Maintainability Prediction Calculations...11-16 Performing a Maintainability Prediction...11-17 Reviewing Maintainability Prediction Calculation Results...11-20 Generating Reports in Relex Maintainability Prediction...11-21
Chapter 12 Life Cycle Cost (LCC)
12-1
Introduction...12-1 Getting Started with LCC...12-1 What is the Goal of an LCC Analysis?...12-1 How Do You Begin an LCC Analysis?...12-1 What Are the Main Steps for Performing an LCC Analysis in Relex?...12-2 How Do You Determine the Structure of Your LCC Analysis?...12-4 Life Cycle Cost Reference...12-4 Relex Sample LCC Files...12-4 LCC File Properties...12-5 Creating/Opening an LCC File...12-6 Entry of Cost Breakdown Structure...12-6 Entry of Equation Definition...12-7 Customizing a Life Cycle Cost Analysis...12-9 Life Cycle Cost Calculations...12-16 Performing an LCC Calculation...12-16 Reviewing LCC Calculation Results...12-19 Generating Reports in Relex LCC...12-19
Chapter 1 Getting Started
Read Me First!
Welcome
Thank you for choosing Relex!
This chapter is intended to get you up and running with Relex in minimal time. Use this “Read Me First” section for installation instructions and quick startup procedures.
Also included in this chapter is a section titled “Introduction to Relex and Reliability”. This section is intended to provide you with background on general reliability topics and procedures and how Relex fits into your analysis tool set.
For details on using the specific Relex modules, refer to the appropriate chapter in your documentation package.
Installation
Hardware Requirements
The minimum requirements to operate Relex are: 486 or greater PC compatible
32MB RAM
75MB Hard Disk Space CD-ROM drive 32-bit Windows
It should be noted that increased memory, processor speed, and hard drive space always improves the performance of any software package.
Installing Relex
Figure 1 - . Relex Installation Options There are two installation modes available when installing Relex.
The Express installation, recommended for most single-user installations, installs Relex with minimal user input.
The Custom installation allows the customization of all available setup options. The Custom installation is required for network installations and recommended only for advanced users and/or system administrators. For additional instructions on custom installations, refer to the section titled “Custom Installation”.
For both installations, Relex requests the location of the Relex files and all associated directories. The primary directory may be read-only, but the other directories must be read/write accessible.
Note: During installation, if you receive the message, “A read only file <file name> was found while
attempting to copy files to the destination location”, select <Yes> to overwrite the file.
Single-User Installation
Follow these steps to install Relex on a stand-alone computer:
1. Startup Windows. If you are installing under Windows NT, you must log on to an account with Administrator privileges. Make sure all applications are closed.
2. Insert the Relex CD. If Setup runs automatically, follow the on-screen instructions. Otherwise, run Setup by selecting Run from the Start menu and entering X:\Setup.exe where X: is the drive letter of your CD-ROM drive. If you install this upgrade in the same directory as a previous Relex version, that previous version will be unusable. 3. After installation is complete, you need to license your Relex package. Refer to the section titled “Licensing
Relex” for instructions on licensing Relex.
Note: If you are installing a single-user version under Windows NT, you must log on to an account
with Administrator privileges to install Relex properly.
Network Installation
Custom Installation
When performing a custom installation, Relex allows you to specify the installation path of certain file folders. These folders or directories are described as follows:
Folder
Description
Files Folder The Files Folder contains all Relex Sample files and supplied Relex files (Report Design files, Derating files, Modes files, etc). This directory is also the default location where new data files are stored. This directory must be read/write accessible.
Relex Libraries Folder
The Relex Libraries Folder contains Library files that are supplied with Relex. It is also be the default location where new Library files are stored. This directory must be read/write accessible.
Auxiliary Files
Folder This folder contains special files that are required to be read/write accessible by Relex. Once the paths for the above folders are selected, you have the option to include or exclude certain files from the installation process. Below is a brief description of each option that can be included or excluded during the installation:
Option
Description
Program Files Files required by Relex to operate. Under this category, specific file types can be included or excluded. (It is advised to install all file types under this category. If selected improperly, error messages and various problems in operating the program may occur.) These file types are:
Demo Libraries. Libraries used by the Relex Demo program. These Libraries are limited versions of the full Relex Libraries.
Data Access Objects Core Components. Required for Relex to operate properly.
Data Access Objects. Required for Relex to operate properly. Shared DLLs. Required for Relex to operate properly. ActiveX Controls. Required for Relex to operate properly.
Report Design Files Report Design files supplied by Relex. If these files are being re-installed, they may overwrite the existing Report Design files, and therefore, overwrite any changes that you have made to Relex supplied Report Design files. Sample Files Sample files used as examples. These files are referred to in the Relex
Tutorial, and provide a good source of reference for new users.
Template Files Template files provide a format by which new files are created. If these files are being re-installed, they overwrite the existing Template files, and therefore, overwrite any changes that you have made to Relex supplied Template files.
Integrated Circuit
Libraries The files that are required to access the Relex Integrated Circuit Library. If these files are being re-installed, they overwrite the existing Relex Integrated Circuit Library files, and therefore, overwrite any changes that you have made to the Relex IC Library.
Option
Description
Libraries existing Semiconductor, Resistor, and Capacitor Library files, and therefore, overwrite any changes that you have made to these Relex supplied files. Non-Electronic
Parts Libraries
The files that are required to access the Non-Electronic Parts Library. FMEA Files The files that are included with Relex FMEA.
Maintainability
Prediction Files The files that are included with Relex Maintainability Prediction. LCC Files The files that are included with Relex Life Cycle Cost.
Network Setup The files required for a network installation. In order for this to work properly, a network version of Relex must have been purchased.
Relex Libraries Installation
When the Relex Libraries are installed, they cannot be edited. If necessary, the Library files can be changed to have read/write access using Microsoft Explorer. Although this is an option, it is advised that changes to Relex Libraries be limited in order to ensure the integrity of the data in these files.
Short File Names Supported
The Relex installation procedure supports both long file names and short file names. During the installation process, Relex analyzes your system to see if it supports long file names. If your system does not support long file names, Relex returns a message stating that short file names will be used for the installation.
Long file names are advantageous because the long name allows for a better explanation of the file contents. For a complete listing of all Relex supplied files and their corresponding long and short filenames, refer to the Docs directory on your Relex CD.
Licensing Relex
Once Relex has been successfully installed, you need to license your Relex package in order to operate Relex on your computer. Until you have received an authorized license, Relex will operate in demo mode. Your Relex license is keyed to a particular computer, so you must license Relex on the computer you are going to be using to operate Relex. For information on licensing Relex, see the installation sheets that you received with your Relex purchase. This information is also available in the Docs directory on your Relex CD.
Starting Relex
Starting Relex is just like starting any Windows program. To start Relex, select Relex from the
Start>Programs menu.
When first starting, Relex displays the Tip of the Day window. This feature provides helpful tips on using Relex. From this window, select the Startup tab. This tab presents several options. When just starting, it may be helpful to select the Open Sample Project option. This automatically opens a supplied sample Relex data file, called a Project file, which you can use to view example data, calculate results and generate reports. You may also select the Create New Project option to open a new Relex Project file that you can use to start your analyses.
Converting from Previous Versions of Relex
Converting Relex Files from Relex Version 6.x
Relex 7 includes many new features and functions that require your current data files to be converted. Before converting your files, create back up copies of all your original files. Once a file has been converted, the converted file can no longer be opened in previous Relex versions.
Relex 7 includes a completely redesigned FMEA module. Because the conversion is not simply a one-to-one type of conversion, you may consider completing your current FMEAs with Relex 6 and then starting new FMEAs in Relex 7. The conversion process will convert your FMEAs, however, you should read the “FMEA” chapter of this manual to become familiar with the new Relex FMEA module to use Relex 7 FMEA.
Converting Project Files
Figure 1 - . Converting Relex 6.x Project Files
Converting Relex Project files from Relex 6 to Relex 7 is a multi-step process that Relex guides you through. The procedure to convert a Project file is as follows:
1. Open the Project that you wish to convert.
2. Relex asks if you are sure you want to convert the Project. Press <Yes>.
3. At this point, you may skip all further questions and have Relex convert your Project file with a set of default conditions. To do this, press <Convert Now!>, and all further steps will be skipped. Otherwise, to go to the next step, press <Next>.
4. For Project conversions, Relex 7 automatically creates a backup copy of your file. By default Relex names this file “6.x Backup of <filename>”. To change the name of the backup file, enter a new name. Press <Next>.
5. In Relex 7, you may choose the information that is stored in your Project file. This allows you to enable only the required components within a Project file to keep file sizes to a minimum. The conversion process displays what information is currently in your Relex 6 Project file, and allows you to select what information you want to include in your Relex 7 Project file. Select the information that you want and press <Next> when complete.
6. In Relex 6, Modes files and Tasks files were based on internal ID generated by Relex. In Relex 7, you assign a unique alphanumeric code to items in these files. The Relex 7 conversion automatically creates new files using these codes from the description fields in your Relex 6 files. You can select how many characters to use for these codes. For example, you may have failure modes in your Relex 6 files such as “Resistor fails open” and “Resistor fails shorted”. In this case, it would be useful to have Relex use the first 25 characters for code creation since the first parts of these modes are identical. If your failure modes are “Open” and “Shorted”, 10 characters may be sufficient. Relex does not duplicate codes, so in the first case, if only 8 characters are used, the first failure mode code would be “Resistor1”, and the second failure mode code “Resistor2”.
7. Relex 7 includes new Input Format files for the configuration of the Parts Table and the FMEA Data tab. Relex 7 automatically converts your Relex 6 Parts Table Format and Relex 6 FMECA Input Format to these new formats.
8. In Relex 6, several files were embedded within the Project file itself. Relex 7 now uses external files so that this data is easily accessible across multiple Projects. These files are now also based on a unique user assigned code instead of a Relex generated internal ID. As described in Step #5, you need to specify how many characters Relex should use when generating these new codes. Enter the names for the new files and the number of characters to use for creating codes. When complete, press <Next>.
9. Press <Finish> to complete the conversion. Relex displays a progress bar during the conversion.
Converting Parts Table and FMECA Input Files
Relex 7 uses a general Input Format file for configuring input forms. To convert Relex 6 Parts Table files and FMECA Input files to this new format, follow these steps:
1. Select File>New>Format>Input Format from the menu and enter the name of the new file.
2. To convert a FMECA Input file, select FMEA for the Table Type. Select Prediction to convert a Parts Table file to an Input Format File for use in a Project file. Select Library or Default to convert a Parts Table file to an Input Format file for use in a Library or Default file.
3. When the file is opened, select Tools>Convert Version 6.x File from the menu. 4. Select the file to be converted.
Converting All File Types Other Than Project, Parts Table, and FMECA Input Files
To convert data files other than Relex Project, Parts Table, and FMECA Input files from Relex 6 to Relex 7, select File>Open from the menu and enter the name of the Relex 6 file. A message appears stating that the data file was created with a previous version of Relex, and asking if you are sure you want to convert it. To convert the file press <Yes>, to cancel the process, press <No>. Please note that you should always save a backup of your files before converting them.
Converting Reliability Prediction Files from Relex Version 5.x
Figure 1 - . Converting Relex 5.x Files
For Subassembly, Library, and Correlation files created with Relex version 5.x, these files must be converted. This feature is accessed by selecting Tools>Convert from the menu in Relex. To convert your Relex 5.x files follow these steps:
1. Backup all of your existing files. 2. Start Relex and select Tools>Convert.
3. In the File Open dialog box, select the Relex 5.x Subassembly, Library, or Correlation files that you wish to convert and press <Open>. Go to Step 4 if you are converting a Library or a Correlation file. Go to Step 5 if you are converting a Subassembly.
4. If you are converting a Library or a Correlation file, Relex immediately begins the conversion, naming the new Library or Correlation file with the same name.
5. If you are converting a Subassembly, Relex allows you to create a new Project or append the assembly to an existing Project. Enter the name of a new Project or enter the name of an existing Project (to add a new assembly
6. Repeat steps 2-5 for any additional files that you wish to convert.
7. When you open the new files, Relex performs all other conversions automatically.
The Relex 5.x conversion process also includes the ability to convert an entire directory of Relex 5.x files. This function automatically converts all contents of the directory into one Relex Project file. To convert an entire directory of Relex 5.x files, follow these steps:
1. Start Relex and select Tools>Convert from the main menu.
2. When the Convert Data File to Relex window appears, select Relex 5.x Directory from the choice list labeled Convert.
3. In the Directory field, use the <Browse> button to browse to the directory of files which is to be converted. 4. Select <OK> when all options are set appropriately.
5. A window appears asking that a Project name be entered for the new Relex file that is created. Enter the new Project name into the field.
6. Select <OK> to perform the conversion.
7. When you open the new Project, Relex performs all other conversions automatically.
Note: Once the Relex 5.x files have all been converted into a Project file, the assemblies that are
created can be copied and pasted into other Project files as needed, or moved within the same Project file.
Basic User Interface
Using Relex is much like using any Windows application. It is first important to notice common features like a standard title bar and minimize and maximize buttons, but Relex also uses common menu configurations and toolbars.
File Management
Like standard Windows applications, the Relex files are managed by using the File option on the main menu. The File option includes the ability to create new files, open existing files, save files, and save files as different names.
Relex Project Files
Project files in Relex are the main storage areas for all data pertaining to your analysis. Relex Project files include Reliability Prediction data, FMEA data, Maintainability Prediction data, RBD data and Fault Tree data. There are sample Project files supplied for review, but you eventually need to create your own Project files.
Figure 1 - . Relex Sample Project File
Relex Value-Added Files
In addition to Project files, there are many different file types used in Relex. Although the Project file in Relex is where the majority of your work is focused, the various other files that are available give you the ability to customize the features of Relex, and provide for performing modular analyses. Value-Added files can range from Library files and Correlation files (which allows you to store data regarding components) to Derating and Default files (which give you more control over the handling and review of data). The Value-Added files in Relex offer additional flexibility to the software. The use of Value-Added files is dependent on the Relex modules enabled. For more information on these various file types, refer to the appropriate chapter in this manual.
Relex Documentation
The Relex Documentation is organized as follows:
Chapter 1 – Getting Started
The “Read Me First!” section is intended to get you up and running with Relex in minimal time. The “Introduction to Relex and Reliability” section is directed towards those who are new to Relex and new to reliability. This document outlines some basics about reliability including a description of Reliability Prediction, Reliability Block Diagrams, Fault Tree, FMEA, Maintainability Prediction, and Life Cycle Cost. This is useful for those who have no direct knowledge of Relex or reliability, and for those readers that are interested in learning more about Relex Software Corporation, the Relex
Chapter 2 – General
This is the general reference chapter for all Relex products. It consists of three main sections that cover operations that are commonly used in Relex including the Relex desktop, right mouse support, conventions, and keyboard options. Also, it includes information on managing files in Relex, and some of the various options available to you in Relex.
Chapter 3 – File Types
This chapter covers the all of the file types used in Relex.
Chapter 4 – Reporting and Graphing
This chapter provides you with information on the reporting and graphing features included in Relex.
Chapter 5 – Importing and Exporting
The CAD Interface module includes the importing and exporting functions of Relex. This chapter outlines the importing and exporting processes in detail.
Chapter 6 – Common Features
This section outlines the use of macros, find and replace, spell check, revision control, and password protection.
Chapter 7 – Reliability Prediction
This chapter provides product specific details on the Relex Reliability Prediction module.
Chapter 8 – Reliability Block Diagram (RBD)
This chapter provides product specific details on the Relex Reliability Block Diagram (RBD) module.
Chapter 9 – FMEA
This chapter provides product specific details on the Relex Failure Mode and Effects Analysis (FMEA) module.
Chapter 10 – Fault Tree
This chapter provides product specific details on the Relex Fault Tree module.
Chapter 11 – Maintainability Prediction
This chapter provides product specific details on the Relex Maintainability Prediction module.
Chapter 12 – Life Cycle Cost (LCC)
This chapter provides product specific details on the Relex Life Cycle Cost module.
Documentation Conventions
There are three categories of conventions used in this manual and the online help system.
General Conventions
Following are General Conventions used in this manual and the online help system.
Convention
Description
keyboard.
<OK> Buttons in windows are designated by <> characters. For example <OK> means to press the “OK” button in the active window.
Press <OK> When the term Press <OK> is used, it means to click the “OK” button with your mouse to perform the action.
Enabled A dialog box option is enabled when a check mark appears beside it, or its button contains a black dot. The absence of a check mark or black dot means the option is disabled.
Choice List Button Refers to the small button to the right of a field that is marked with an arrow. When pressed, this button displays a choice list of all valid options for the selected field.
Mouse Conventions
Relex uses both mouse buttons. It is assumed that you have programmed the left mouse button as the primary mouse button. Whenever a procedure requires you to use the secondary mouse button, it is referred to as the right mouse button.
Menu Conventions
When referring to menu and sub-menu items in Relex, this guide directs you one step at a time through each menu choice. For instance File>New>Project means to select File from the main menu at the top of the window, and then select the sub-menu item titled New, and finally the sub-menu item titled Project.
Keyboard Conventions
Convention
Description
Accelerator Keys Many features that can be accessed from the menus have accelerator keys assigned to them. Accelerator keys are keyboard key combinations that offer shortcuts to frequently used functions. The accelerator keys are shown on the Relex menu. For example, when you select the File menu, you find the notation of Ctrl+S to the right of the Save menu option. This notation shows that Ctrl+S is an accelerator key for the File>Save command. Ctrl+S can be used rather than selecting File>Save from the menu as a shortcut.
CTRL+F1 Carrying out commands or procedures frequently involves pressing two or three keys together. For example <Ctrl> + <F1> means to hold down the <Ctrl> key and press the <F1> key simultaneously.
<Arrow Keys> The notation <Arrow Keys> means the keys marked with either left/right or up/down arrows on your keyboard.
Where to Find Help
Online Help
Information included in the online help system includes descriptions of all fields required for the completion of reliability analyses, helpful hints, and step-by-step instructions for completing tasks. There are three basic techniques used to access the Relex help system.
You can access the Relex online help system by selecting Help>Relex Help from any menu. This particular method for activating the help system accesses the standard help interface that consists of three tabs. Each of these three tabs provides a different way to access information in the Relex help system. The Contents tab divides the information in the help system by chapters (similar to the organization of the printed documentation). The Index tab is an alphabetic listing of all of the main topics in the Relex help system. Lastly, the Find tab is a simple search feature that allows you to find any word or phrase in all of the text in the online help system. Select the tab that best suits your needs.
On most dialog boxes, you have the ability to click the <Help> button. Upon selecting the <Help> button, the Relex online help system displays the specific online help topic associated with the current window.
Pressing the <F1> key brings up the context sensitive help for the current activity.
Printed Documentation
The printed documentation includes all information available through the online help system. It is provided in printed form for easy access.
Contacting Relex Technical Support
If you find that you are unable to find all of the answers that you are looking for, don’t hesitate to contact the Relex Technical Support Department.
Mail Address: Relex Software Corporation 540 Pellis Road
Greensburg, PA 15601 USA Phone Number: 724-836-8800
Fax Number: 724-836-8844
Our Home Page: http://www.relexsoftware.com
Support E-Mail: [email protected]
When contacting Technical Support, have the following information available:
Product version number, found by choosing Help>About Relex from the main menu in Relex (specific incremental version numbers can also be found by accessing the File Versions tab under Help>About Relex).
A copy of this manual for possible cross-reference. The type of computer hardware you are using. The version of your operating system.
Exact wording of any messages that appear on your screen.
Introduction to Relex and Reliability
The level of information and description in this section is geared towards those who are new to reliability, and also to those who feel that a review of reliability and an introduction to Relex would be helpful. Below is a list of the main chapters in this document along with a brief description of each.
Reliability Techniques – This section provides a list of several common reliability techniques along
with a brief explanation of each technique and its use.
Introduction to Reliability Prediction – Describes the basics of performing a reliability prediction
analysis, and includes reviews of the models used and the general calculations that are performed.
Introduction to Reliability Block Diagram (RBD) – Describes the benefits of modeling a system
using a reliability block diagram, and reviews the various redundancy types that might be included.
Introduction to FMEA – Describes the basics of performing a failure mode and effects analysis
(FMEA). Includes a brief description of the steps involved in performing a FMEA or FMECA, and an outline of some of the calculations that may be required.
Introduction to Fault Tree – Explains the basics of a fault tree analysis, and defines the differences
between qualitative and quantitative analyses.
Introduction to Maintainability Prediction – Outlines the purpose of performing a maintainability
prediction analysis, and explains simple steps that are to be followed.
Introduction to Life Cycle Cost (LCC) – Provides a definition of a Life Cycle Cost analysis along
with a common sense explanation of how LCC relates to reliability and maintainability prediction analyses.
Who, What, and Why?
Who is Relex Software Corporation?
Relex Software Corporation was incorporated in 1986 as Innovative Software Designs, Inc. Our goal from the start was to combine an expertise in reliability analysis with expertise in software engineering to produce a set of superior, user-friendly reliability analysis software tools.
The first product, Relex 217, a DOS-based reliability prediction package based on MIL-HDBK-217, was released in 1987. The name Relex was derived from the term Reliability Excellence. The
popularity of this product lead to the development of Relex Bellcore, Relex FMEA/FMECA and Relex Maintainability Prediction in the following years.
In 1993, the entire Relex product line was redeveloped for Windows 3.1. This release solidified Relex's reputation as a leader in the development of reliability software packages.
In 1997, the entire Relex product line was again redesigned for the Windows 95 and Windows NT platforms. At this time, the Relex products were integrated into a singular modularized application. With the introduction of Relex Version 6.0, our Windows 95/NT product, Relex Software introduced our innovative integrated solution concept.
In 1997, the Relex Reliability Block Diagram tool was introduced to the product suite. Also in 1997, we began our comprehensive, well-received training courses on reliability engineering and using Relex.
In 1998, our name was officially changed from Innovative Software Designs, Inc. to Relex Software Corporation to reflect our ongoing commitment to Reliability Excellence. In 1998, we released the Relex Life Cycle Cost package.
Relex Software Corporation is headquartered in Greensburg, PA, a suburb of Pittsburgh, PA. All of our products are developed and supported in-house. All software development, technical support and US sales are handled from our corporate office. Other sales offices are in place throughout the world, to support our international network of users.
Relex has grown into a worldwide leader in reliability analysis tools. Our reputation for technical excellence is unmatched. Our high quality, state-of-the art product line and unparalleled customer
What is Relex?
Relex is a completely integrated set of reliability analysis tools. Relex performs reliability predictions and analyses on electrical, electronic, mechanical, and electro-mechanical equipment. Relex also provides the ability to perform Design and Process FMEAs, Reliability Block Diagrams, Fault Tree Analyses, Maintainability Predictions, and Life Cycle Cost Analyses. Relex provides all these
capabilities in a unique, fully integrated framework. Relex has become an industry standard tool set for performing R&M analyses.
What is Reliability?
The textbook definition for reliability is the probability that an item will perform a required function without failure under stated conditions for a stated period of time. As illustrated by this definition, reliability is a very broad term that focuses on the ability of a product to perform its intended function. In general, the process of performing a reliability analysis can actually include a number of different analyses on a product in order to anticipate how reliable that product is. It is then possible to anticipate the effects of design changes and corrections in order to improve reliability. The different analyses that can be performed are all related and analyze different aspects of product reliability. Each of these different analyses looks at a product from a different angle in order to determine possible problems, and assist in analyzing corrections and improvements. Some types of reliability analyses include reliability predictions, failure mode and effect analyses, and fault tree analyses.
Why is Reliability Important?
There are several reasons why analyzing reliability is important; a few of the most common are outlined below.
Reputation
A Company’s reputation is very closely related to the reliability of their products. If the products that are manufactured are very reliable and perform the required function without failure under stated conditions for a stated period of time, then the reputation of the company experiences a direct benefit. Therefore, the more reliable a product is, the more likely a company is to have a very favorable reputation.
Warranty Costs
Oftentimes, a company may begin to produce a product but never has performed a serious reliability analysis on that product. Unfortunately, after the product is distributed, it could be found that it is not performing its required function without failure for a stated period of time. If the product fails prematurely, there may be an issue of warranty or replacement costs involved. These types of premature failures inevitably cause a negative impact on profits. This type of problem often gains negative attention, and introducing reliability analysis measures can be an important step in taking corrective action. Even if the only initial purpose of performing a reliability analysis is to decrease costs alone, the end result is a better product that is more reliable.
Future Business
In order to keep customers, it is very important that you remain satisfied. A concentrated effort towards improved reliability proves to customers that a manufacturer is serious about their product, and committed to customer satisfaction. Needless to say, this type of attitude has a positive impact on the stability of future business.
a reliability prediction that meets specific failure rate or MTBF requirements
a failure mode and effects analysis performed and presented with specific report formats a life cycle cost analysis analyzing all future costs including maintenance and spare parts Even if these issues are not contractually required, performing any or all of these tasks is beneficial to any product manufacturer.
Cost Analysis
Companies are finding the ability to perform a cost analysis to be a very valuable analysis tool. In order to illustrate the cost effectiveness of their products, a manufacturer may take reliability data regarding the product and combine it with other cost information in order to illustrate the overall life cycle cost of their products. This type of analysis can prove that although the initial cost of the product may be higher, the overall lifetime cost of that product is actually lower than a competitor’s because their product requires fewer repairs and/or maintenance.
Reliability Techniques
Now that you have received a brief introduction to Relex Software Corporation, Relex, and reliability, it is important to understand some of the many different reliability techniques that can be performed. As mentioned earlier, the definition of reliability is very broad. Reliability is not one type of analysis. Instead, reliability can be examined by a number of different types of analyses.
Reliability Prediction
A Reliability Prediction is one of the most common forms of reliability analyses. A reliability
prediction is the analysis of parts and components in an effort to predict the rate at which an item fails. A reliability prediction is usually based on an established model. Common models are MIL-HDBK-217 or Telcordia (previously Bellcore). These models provide procedures for calculating failure rates for components. Failures rates are calculated by gathering information regarding components (like stress data, quality information, temperature, and environment factors), and performing a calculation based on standard equations.
The failure rates that are generated from a reliability prediction can then be used in other reliability analyses to further review the reliability of the item. For instance, failure rates can be used in a FMEA, a maintainability prediction, a life cycle cost analysis, or a reliability block diagram. In Relex, all of these different analyses are integrated together so that data, like failure rate, is automatically shared from one analysis type to another.
Reliability Block Diagram (RBD) Analysis
A Reliability Block Diagram is a tool for analyzing more complex systems and configurations. When performing a reliability prediction analysis, failure rates for components, assemblies, and systems are calculated. However, the models used for reliability prediction assume that all components and assemblies are configured in series, and failure rate calculations are based on that assumption. However, many systems are designed with various redundancies of key assemblies in order to improve the overall reliability of that system. The analysis of these types of redundancies requires the use of sophisticated mathematical algorithms. Relex RBD is a complete graphical block diagram evaluator that is integrated with the other Relex modules for the complete analysis of these types of
configurations.
Relex RBD analytically analyzes systems to compute failure rate, MTBF, reliability and availability, unreliability, and unavailability. For more complex configurations, when an analytical solution is not
process of determining the number of spares required for a system for optimum availability and minimal cost.
Failure Mode and Effects Analysis (FMEA, FMECA)
A Failure Mode and Effects Analysis (also referred to as a FMEA or FMECA) is a bottoms up approach to analyzing system design and performance. To begin a FMEA, a particular level of a system is analyzed. This can be the component level (referred to as a piece part FMEA) or at a higher level (referred to as a functional FMEA). For each lowest level, a list of potential failure modes is generated. Effects of each potential failure mode are then determined.
For example, perhaps you are performing a piece part FMEA on a computer monitor. One component in that computer monitor might be a capacitor. You have determined that there are 2 potential failure modes for the capacitor, and they are that the capacitor could fail open or it could fail short. If the capacitor fails open, the effect might be that the monitor appears with wavy lines. However, if the capacitor fails short, the effect might be that the monitor goes completely blank. In this case, if the capacitor fails short and the monitor goes blank, that failure mode could be considered more severe or critical than if the capacitor fails open and wavy lines appear. In this case, you attempt to find ways to prevent these failures from happening or lessen the criticality.
A FMEA can use failure rate calculations that were performed during the reliability prediction portion of an analysis to determine probability of occurrence. Failure rate is a value describing how often a component or assembly fails. In a FMEA, failure rates are used to compute mode criticality, or the probability that a particular failure mode is actually going to occur.
Like reliability prediction analyses, FMEAs and FMECAs are performed based on models too. There are primarily three types of FMEAs or FMECAs. First, there are MIL-STD-1629 FMECAs, which are based on the military standard for performing FMEAs. Second, there are automotive style FMEAs, which are based on various automotive FMEA models. In addition, there is a third standard, SAE ARP5580 that outlines a general approach for FMEA analysis. Relex supports all of these accepted FMEA/FMECA standards.
Fault Tree Analysis
A Fault Tree analysis is yet another reliability technique. Like a FMEA, a Fault Tree analysis focuses on failures. However, a Fault Tree analysis is a top down approach (rather than a bottom up approach like FMEA). A Fault Tree analysis is a graphical analysis that assumes a system failure, and traces down through the hierarchy to find what could have caused it.
Very often, in order to be complete, both a Fault Tree analysis and FMEA are performed on a system. This enables every possible failure to be analyzed and corrected or minimized. In Relex, you can perform a fault tree analysis along with a reliability prediction and FMEA on the same system in one integrated software package. Failure modes from a FMEA and items from a reliability prediction can be included in a fault tree analysis.
Maintainability Prediction
Maintainability Predictions provide calculated information regarding various aspects of maintenance. The goal of performing a maintainability prediction is to determine the amount of time required performing repairs and maintenance tasks. Maintainability predictions enable you to analyze how long a repair takes in the case of a system failure.
Relex provides you with the ability to perform a maintainability prediction based on accepted standards. Relex supports MIL-HDBK-472 Procedures 2, 5A and 5B, and generic maintenance procedures. Relex Maintainability Prediction automatically performs all the maintenance-related calculations. For example, a common maintainability parameter is Mean Time to Repair (or MTTR). This is the most common measurement for a maintainability prediction, and it is the mean time to
that an item is in an operable state at any time, and is based on a combination of failure rate and MTTR. Since Relex Maintainability Prediction is integrated with Relex Reliability Prediction, these types of calculations can be performed.
Life Cycle Cost Analysis
A Life Cycle Cost Analysis is a method of calculating the cost of a system over its entire life span. The analysis of a typical system could include such costs as System Planning and Concept Design,
Preliminary System Design Cost, Design and Development Costs, Product Costs, Maintenance Costs, and Disposal Costs. This type of analysis often uses values calculated from other reliability analyses like failure rate, cost of spares, repair times, and component costs.
A company could use a life cycle cost analysis to determine warranty costs, for instance. This type of analysis could be based on anticipated failures, repair times, and costs of repairs. Many companies are finding that a life cycle cost analysis is a valuable tool during the design phase of a project in order to determine the most cost-effective solutions before substantial costs are incurred.
Introduction to Reliability Prediction
Generally, the first step to analyzing reliability is to perform a reliability prediction analysis. This section provides some background information on performing reliability predictions.
Measures of Reliability
When performing a reliability prediction analysis, there are several calculated values that provide measures of reliability. These values include failure rate, MTBF, reliability and availability.
Failure Rate
Failure rate is defined as the rate of occurrence of failures. This value is normally expressed as failures per million hours, but it can also be expressed as a FIT Rate (Failures in Time) or failures per billion hours. Failure rate is basically the anticipated number of times that an item fails in a specified period of time. For example, if a component has a failure rate of 2 failures per million hours, then it is
anticipated that the component fails 2 times in a million hour time period.
A calculated failure rate is generally based on an established reliability prediction model (like MIL-HDBK-217 or Telcordia). Calculations are generally based on component data such as temperature, environment, and stress. Calculated failure rates for assemblies are a sum of the failure rates for components within the assembly.
A component manufacturer may sometimes provide a specified failure rate. This failure rate is usually based on field or laboratory test data. Similarly, a manufacturer can also provide a specified failure rate for an assembly.
MTBF
MTBF stands for Mean Time Between Failures. MTBF is the inverse of the failure rate for constant failure rate systems. As a simple summary, MTBF can be described as the number of hours to pass before a failure for a component, assembly, or system. For example, if a component has a failure rate of 2 failures per million hours, then the MTBF would be the inverse of that failure rate. Therefore, 1,000,000 divided by the failure rate of 2 would calculate as an MTBF of 500,000 hours (1,000,000 / 2 = 500,000). This value states that the component is to fail approximately every 500,000 hours.
It is important to understand that MTBF is not a definite value. If a component has an MTBF of 8,000 hours, it means that on average that component fails every 8,000 hours.
occurs. For non-repairable systems, there is no repair. Therefore, in the lifetime of a device, the device fails once. A typical example is a satellite in orbit. If a satellite fails, generally no one is sent to repair the device. Instead, it may be that a new satellite would be launched to replace the failed unit. MTTF represents the average time until this failure occurs.
Reliability
Another important measure is termed reliability. The definition of reliability is the probability that an item will perform a required function without failure under stated conditions for a stated period of time. When calculating reliability, the important factor is time. The calculation of reliability takes into account a value called mission time, or time of operation.
For constant failure rate systems, the equation for the calculation of reliability is: R = e-t.
Where t is the mission time, and is the failure rate in failures per hour. Reliability is expressed as a probability value (a value between 0 and 1). A reliability value of 0 means that your system definitely fails within its mission time. A reliability value of 1 means that your system does not fail during its operational lifetime.
Reliability is the probability that an item will perform its required function for a stated period of time. As an example, perhaps you have calculated an MTBF for a device to be 500,000 hours. In this case, it would be anticipated that the device would last 500,000 hours before failure. If the mission time for the device was 100 hours (in other words, it needs to function for a 100 hour mission), the probability that the device would last its 100 hour mission would be very high. However, if that same device had an MTBF of 50 hours, and a mission time of 100 hours, the probability that the device would complete its 100-hour mission would be significantly lower.
Availability
The calculated value of availability takes the analysis yet one step further. Availability is defined as the probability that an item is in an operable state at any time. Like reliability, availability is a probability value ranging from 0 to 1. Unlike reliability, however, availability takes into account repairs.
When repairs are applicable for a device, a value referred to as MTTR can be applied. MTTR stands for Mean Time To Repair, and basically refers to the downtime required to repair a device. Again, like failure rate, MTTR can be calculated (through the use of a maintainability prediction), or it can be specified. The value of MTTR is then used in the calculation of availability to determine the probability that an item will be operable when needed.
The equation for the calculation of steady-state availability is: Availability = MTBF/(MTBF + MTTR) As another simple example, imagine that you have a device with an MTBF of 50 hours. This, again, means that the device may be operational (without failure) for approximately 50 hours. When the unit fails, it then needs to be repaired. If the MTTR (or time required for repair) is 10 hours, then that means that every time the unit fails, it needs to be down for about 10 hours before it can be repaired and put back online.
Systems with high MTBF in conjunction with low MTTR have high availability. As MTBF decreases and/or MTTR increases, system availability decreases. This means that at a certain time, your system is more likely to be down, or in a failed state. This becomes especially important for critical systems. The value of availability also becomes very important when analyzing various modeling options and the implementation of redundant components.
Reliability Prediction Models
When performing a reliability prediction analysis, the calculations that are performed are based on established reliability models. For electronic components, the two most popular and widely accepted
MIL-HDBK-217
MIL-HDBK-217 was the original standard for reliability predictions. It was designed to provide reliability math models for nearly every conceivable type of electronic device. It is used by both commercial companies and the defense industry, and is accepted and known worldwide.
MIL-HDBK-217 includes the ability to perform a parts count analysis or a part stress analysis. A parts count analysis is not as detailed as a part stress analysis, and is normally used early in a design when detailed information is not available, or a rough estimate of failure rate is all that is required. A part stress analysis takes into account more detailed information regarding the components, and, therefore, offers a more accurate estimate of failure rate.
Telcordia (Bellcore)
The Telcordia reliability prediction model was originally developed by AT&T Bell Labs. Bell Labs modified the equations from MIL-HDBK-217 to better represent what their equipment was
experiencing in the field. The main concepts between MIL-HDBK-217 and Telcordia are very similar, but Telcordia added the ability to take into account burn-in, field, and laboratory testing. This added ability made the Telcordia standard very popular with commercial organizations. In 1998, Bell Communications Research (Bellcore) became Telcordia Technologies.
Telcordia also supports the ability to perform parts count or part stress analyses, but in Telcordia, these different calculations are referred to as calculation methods. Each method is designed to take into account different information. This information can include stress data, burn-in data, field test data, or laboratory test data.
Although the use of calculation methods originated with the Telcordia model, Relex allows you to implement these calculation methods with the Telcordia or MIL-HDBK-217 models.
Mechanical Model
The “Handbook of Reliability Prediction Procedures for Mechanical Equipment” (NSWC-98/LE1) provides reliability prediction models for mechanical components. This model, which was developed under the direction of the United States Navy, provides models for various types of mechanical devices including springs, bearings, seals, motors, brakes, and clutches. This standard is currently the only one of its kind.
Reliability Prediction and the Bathtub Curve
Oftentimes, when discussing reliability prediction, the bathtub curve is used. It is important to understand the bathtub curve, and how it applies to the calculation of failure rate.
The bathtub curve is a simple graph of failure rate versus time that illustrates the failure rate tendencies for the life span of an item. During the life span of an item, the rate at which that item fails may vary as it goes through the various phase of life. The three phases of life that are compared on a bathtub curve are the infant mortality region, the wear-out region, and the constant failure rate region. Each of these phases represents a different phase in the life of an item, and each phase can represent very different tendencies with regard to failures.
Infant Mortality Region
The infant mortality region (sometimes referred to as a region of DFR or Decreasing Failure Rate) is typically a curve, which begins with high failures initially, but drops drastically during the infancy of an item. These high failures are typically caused by poor design and are generally eliminated, as the design becomes mature.
Wear-Out Region
The wear-out region (sometimes referred to as a region of IFR or Increasing Failure Rate) is typically a curve that is very low and consistent initially, but begins to increase drastically as the item begins to age. Fatigue or wear-out typically causes these failures.
Constant Failure Rate Region
The constant failure rate region (CFR) is typically a flat graph representing the constant failure rate of an item. Failures in the constant failure rate region are typically caused by stress. Constant failure rate is the type of failure rate calculation performed when using the MIL-HDBK-217 and Telcordia calculations. Bathtub Curve Time Fa ilu re R at e Infant Mortality Wear-Out Constant Failure Rate Bathtub
The Bathtub Curve
The combination of these three curves produces the bathtub curve. The bathtub curve is a curve which takes all three of these curves into account. Therefore, as an item goes through all of the phases of life, all three of these phases would be taken into account.
It is important to note again, however, that the calculations that are performed by the calculation models (MIL-HDBK-217 and Telcordia) do not consider all of the phases of life included in the bathtub curve. The calculation models are based on the constant failure rate region of the curve. This is the portion of the curve in which a product spends most of its lifetime.
The Telcordia calculation model provides the ability to calculate a first year drop out value, which is a summary of infant mortality. This calculation, however, is a separate analysis, and is not included in the failure rate calculations provided by the Telcordia model. In Relex, you are able to calculate first year drop out using both the MIL-HDBK-217 and Telcordia models, but again, this is a separate analysis and is not included in the failure rate calculations.
Introduction to Reliability Block Diagram (RBD)
As system configurations become more complex, more complex calculation methods are required to calculate values like failure rate, MTBF, reliability, and availability. Performing these complex calculations is the purpose of a Reliability Block Diagram analysis.